1. Field of the Invention
This invention relates generally to well stimulation and is more particularly concerned with an improved method of well stimulation by explosive fracturing.
2. Description of the Prior Art
In view of the extensive and continuing depletion of existing gas and oil reserves there has developed an intensive search for methods by which wells can be stimulated so as to foster additional and economic recovery of valuable fossil fuels therefrom. As employed herein, the term "well stimulation" refers to any method employed to enlarge or create new flow fissures in a downhole fuel producing or "pay" formation. Generally speaking, three broad categories of well stimulation techniques are known, each of which bears certain disadvantages.
Hydraulic fracturing represents one of these categories and is presently widely practiced. In hydraulic fracturing a liquid is injected into the well bore under relatively enormous pressure, thereby to cause splitting and fracturing of the "tight" pay formation. This method finds particular use with respect to sandstone pay formations which are not normally sufficiently amenable to stimulation by means of acidification techniques. While the principal purpose of the liquid employed in hydraulic fracturing is to act as a pressure transfer agent and to thereby transmit the pressure generated at the surface of the well site to the downhole formation, said liquid is also often additionally employed as a carrier for sand or other particulate solids. These solids are conveyed by the liquid into the fissures caused by the hydraulic fracturing and thereafter serve to stabilize the fractured formation and to ensure maintenance of the freshly opened fissures. Typical hydraulic liquids comprise refined oil, crude oil, salt water, acids, emulsifiers and other additives. Principal disadvantages of well stimulation by hydraulic fracturing lie in the expense involved in providing the various and complex equipments required to generate the relatively enormous downhole hydraulic pressures, which may exceed 10,000 p.s.i., and in the safety hazards associated with handling of such relatively enormous captive pressures. Too, hydraulic fracturing is usually a relatively lengthly process to undertake.
Another broad category of well stimulation technique resides in acid treatment of susceptible pay formations. Depending upon the nature and composition of the formation, one or more acids are pumped downhole to the formation and, upon contact therewith, cause channeling and fissuring by chemical reaction. Acid treatment well stimulation techniques find fairly extensive use with respect to pay formations composed of limestone or dolomite which, as a result of their composition, are especially susceptible to hydrochloric acid attack. Various other acids and acid treating formulations can be employed. For instance, hydrofluoric acid and mixtures thereof with hydrochloric acid are often employed when the producing formation to be stimulated comprises clay or sandstone or wherein a portion of the overall stimulation process is directed to the removal of mud from the pore space about the well. Rheological acid compositions are also employed and are generally introduced into the well as a fluid liquid. At the formation site, however, a rheological acid composition tends to set up as a viscous mass, thereby to retard its chemical action until such time as it has found its way back into the tight formation. A major problem usually associated with well stimulation by acid treatment resides in the requirement that the spent acid be periodically or continuously removed from the formation and replaced by fresh acid. This, of course, requires that the spent acid be swabbed or pumped out of the well and that suitable provisions be made for the disposal thereof. Further, should the acid treating agent be left downhole, it can substantially reduce the service life of the pump and other equipment associated with the well.
The third general category of well stimulation technique known in the art, with which category the present invention is associated, is known broadly as explosive fracturing. Explosive fracturing is probably the most venerable of well stimulation techniques and is usually achieved by placing an explosive charge downhole and detonating it so as to shatter the tight pay formation and thereby permit the oil or other fossil fuel of interest to flow through the rubble to the well. Historically, the first methods of explosive fracturing involved the use of pure nitroglycerin which, of course, is an extremely dangerous and sensitive explosive. This problem has been mollified somewhat by the advent of safer explosives which are generally lowered into the well in combination with timed detonators. More recent developments with respect to explosive fracturing techniques involve the use of explosive liquids which are pumped into the pores of the pay formation and are thereafter detonated. Unfortunately, such liquid explosives are also often of a critical compositional nature and are overly sensitive to shock, static electricity, heat and the like. Several serious accidents have already been experienced in association with their use. Atomic explosives have been experimentally employed in fracturing wells and some successes have been had in creating massive fracturing of tight pay formations by this technique in gas wells located in New Mexico and Colorado. Obviously, however, the use of atomic or thermonuclear charges is, as yet, extremely expensive for this purpose and additional safety problems are incurred with respect to proper and safe disposition of radioactive wastes. Finally, the use of explosive fracturing techniques of the prior art in attempting to stimulate a well can often result in substantial downhole cave-ins of the well, thereby choking it with debris. Thus, when explosively fracturing a well in accordance with prior art practices, it is often necessary to remove debris by such ancillary techniques as sand bailing or backflushing of the well bore with a pumped carrier liquid. In accordance with the present invention, however, many of the problems associated with prior art explosive fracturing stimulation techniques have been solved or at least substantially ameliorated.